A monodromy criterion for extending curves

Finite monodromy of some families of exponential sums

Given a prime $p$ and an integer $d>1$, we give a numerical criterion to decide whether the $\ell$-adic sheaf associated to the one-parameter exponential sums $t\mapsto \sum_xψ(x^d+tx)$ over ${\mathbb F}_p$ has finite monodromy or not, and work out some explicit cases where this is computable.

Geometric criteria for tame ramification

We prove an A'Campo type formula for the tame monodromy zeta function of a smooth and proper variety over a discretely valued field $K$. As a first application, we relate the orders of the tame monodromy eigenvalues on the $\ell$-adic cohomology of a $K$-curve to the geometry of a relatively minimal $sncd$-model, and we show that the semi-stable reduction theorem and Saito's criterion for cohomological tameness are immediate consequences of this result. As a second application, we compute the error term in the trace formula for smooth and proper $K$-varieties. We see that the validity of the t…

从代数簇光滑纤维化的开子集扩张态射的一个判据

A criterion for extending morphisms from open subsets of smooth fibrations of algebraic varieties

本文研究代数簇光滑纤维化中开子集上的态射扩张问题，给出一个类似 Zariski 主定理的充分条件，以保证到真态射目标的 $S$-态射可整体扩张。

Monodromy of stable curves of compact type: rigidity and extension

Let ${\cal M}_{g,n}$, for $2g-2+n>0$, be the moduli stack of $n$-pointed, genus $g$, smooth curves. For a family $C\to S$ of such curves over a connected base and a geometric point $ξ$ on $S$, the associated monodromy representation is the induced homomorphism $π_1(S,ξ)\toπ_1({\cal M}_{g,n},[C_ξ])$ on algebraic fundamental groups. It is well known that, if $S$ is irreducible, reduced and locally of finite type over a field $k$ of characteristic zero, the fibre $C_ξ$ and the corresponding monodromy representation determine the relative isomorphism class of the family. In the first part of the p…

判别式补集基本群的拓扑单值核

Topological monodromy kernels for fundamental groups of discriminant complements

本研究拓展Kuno与Carlson-Toledo的方法，证明光滑复代数曲面线性系统的拓扑单值表示像为有限指数时核无限，且平面曲线情形下核含非阿贝尔自由群，结论适用于光滑环面曲面等。

A Kulikov-Type Classification Theorem for a One Parameter Family of K3-Surfaces Over a p-ADIC Field and a Good Reduction Criterion

In this paper, we prove a $p$-adic analogous of the Kulikov-Persson-Pinkham classification theorem [Persson:1981wp] for the central fiber of a degeneration of $K3$-surfaces in terms of the nilpotency degree of the monodromy of the family. Namely, let $X_K$ be a be a smooth, projective $K3$-surface which has a minimal semi-stable model $X$ over $\mathcal O_K$. If we let $N_{st}$ be the monodromy operator on $D_{st}(H^2_{et}}(X_{\overline K},\mathbb Q_p))$, then we prove that the degree of nilpotency of $N_{st}$ determines the type of the special fiber of $X$. As a consequence we give a criterio…

A cut-by-curves criterion for overconvergent of $F$-isocrystals

Let $X$ be a smooth scheme over a finite field. It is conjectured that a convergent $F$-isocrystal on $X$ is overconvergent if its restriction to every curve contained in $X$ is overconvergent. Using the theory of étale and crystalline companions, we establish a weaker version of this criterion in which we also assume that the wild local monodromy of the restrictions to curves is trivialized by pullback along a single dominant morphism to $X$.

Explicitly Extending Frobenius Splittings over Finite Maps

Suppose that $π\: Y \to X$ is a finite map of normal varieties over a perfect field of characteristic $p > 0$. Previous work of the authors gave a criterion for when Frobenius splittings on $X$ (or more generally any $p^{-e}$-linear map) extend to $Y$. In this paper we give an alternate and highly explicit proof of this criterion (checking term by term) when $π$ is tamely ramified in codimension 1. Some additional examples are also explored.

Vanishing cycles, plane curve singularities, and framed mapping class groups

Let f be an isolated plane curve singularity with Milnor fiber of genus at least 5. For all such f, we give (a) an intrinsic description of the geometric monodromy group that does not invoke the notion of the versal deformation space, and (b) an easy criterion to decide if a given simple closed curve in the Milnor fiber is a vanishing cycle or not. With the lone exception of singularities of type $A_n$ and $D_n$, we find that both are determined completely by a canonical framing of the Milnor fiber induced by the Hamiltonian vector field associated to f. As a corollary we answer a question of …

On logarithmic extension of overconvergent isocrystals

In this paper, we establish a criterion for an overconvergent isocrystal on a smooth variety over a field of characteristic $p>0$ to extend logarithmically to its smooth compactification whose complement is a strict normal crossing divisor. This is a generalization of a result of Kedlaya, who treated the case of unipotent monodromy. Our result is regarded as a $p$-adic analogue of the theory of canonical extension of regular singular integrable connections on smooth varieties of characteristic 0.

超越哈纳克阿米巴边界

Beyond Harnack Amoeba Boundaries

该研究构造了非环面等价于简单哈纳克曲线的平面曲线，通过单项式覆盖得到无限族，分类了相关牛顿多边形，证明阿米巴轮廓无法确定曲线的实结构等性质。

On symmetric products of curves

Let C be a smooth complex projective curve of genus g and let X be its second symmetric product. This paper concerns the study of some attempts at extending to X the notion of gonality. In particular, we prove that the degree of irrationality of X is at least g-1 when C is a generic curve, and that the minimum gonality of curves through the generic point of X equals the gonality of C. In order to produce the main results we deal with correspondences on the k-fold symmetric product of C, with some interesting linear subspaces of \mathbb{P}^n enjoying a condition of Cayley-Bacharach type, and wi…

曲线正则模型上 torsor 的延拓

Extending torsors over regular models of curves

本文研究曲线正则模型上带点 torsor 的延拓问题，通过对数 Picard 函子建立等价刻画并给出延拓判据，推广 Chiodo 关于 Néron 模型挠子群的准则，最后以超椭圆曲线为例说明方法。